The present invention relates to a new and improved method for changing the secondary cooling during the continuous casting of metals, typically steel, especially upon changing the section or format, wherein the strand surface is impinged by jets of cooling agent transverse to the strand and which cooling agent jets are produced by spray nozzles, and upon changing of the format the perpendicular distance of the nozzles from the strand surface is altered and thereby the cooling, and this invention also pertains to a new and improved construction of apparatus for the performance of the aforesaid method.
During the continuous casting of steel the strand emanating from the continuous casting mold is directly impinged with cooling agent in the thereafter following cooling zones. The cooling agent or medium, which is usually water, is sprayed by means of spray nozzles upon the strand surface, the nozzles being arranged in spray planes located generally perpendicular to the direction of travel of the cast strand.
At an overwhelming number of continuous casting installations a number of sections or formats are cast at the same installation. In order to accommodate the secondary cooling to the different strand widths and the different qualities of strands which are to be cast, it is already known to the art to shutdown individual nozzles or nozzle groups. This shutting-down of nozzles during the operation is associated with the drawback of calcification of the nozzle openings owing to the action of the heat, leading to irregularities in the cooling when such nozzles are again switched-on.
According to another prior art cooling apparatus one nozzle is arranged in each spraying plane per strand side. This nozzle is constructed such that the strand surface impinged by the cooling agent extends over the entire width of the strand. If the shape or format, i.e. the cross-sectional dimensions of the strand are changed, then the spacing of the individual nozzles with respect to the strand side is adjusted such that the impinged strand surface again covers the entire strand width. When increasing the strand dimension the nozzle must be moved away from the strand and when reducing the strand dimension the nozzle must be moved towards the strand. Appropriate devices are provided for altering the perpendicular distance of the nozzle from the strand surface. The use of nozzles possessing different spray characteristics should thus render possible the accommodation to different cooling conditions and insure for a uniform cooling over the entire strand width.
In order to be able to spray the entire strand width by means of a single spray nozzle, the spacing of this nozzle from the strand surface, especially in the case of very wide slabs, must be chosen to be large. This large spacing requires a high pressure of the coolant or cooling agent in the infeed line leading to the nozzle. Nonetheless the average impact pressure of the cooling agent at the strand surface is relatively small due to the great distance from the nozzle to the surface of the strand. Since with decreasing impact pressure the cooling effect or action is likewise reduced this constitutes a considerable drawback. Normally, the cooling agent is introduced into the intermediate spaces between the guide elements, for instance the strand guide rolls, so as to impinge upon the strand surface. A part of the cooling water runs-off in the form of water droplets over such guide elements especially at the vertical portion of the cooling zone. Since the nozzles are located externally of the guide elements owing to their large spacing from the strand surface, the cooling agent jet, i.e., the spray fan or jet, is considerably disturbed by the water droplets which are flowing off. In the case of a strand width in the order of for instance 2400 millimeters, the spacing between the guide rollers only amounts to about 40 millimeters, so that the nozzles must be fabricated with extreme accuracy in order to produce a spray fan or jet which permits the coolant to pass exactly between the rollers. Also stringent requirements are placed upon the alignment accuracy of the nozzles, because otherwise the spray jets partially, and in some instances, entirely impinge upon the guide rollers instead of upon the surface of the strand. In these instances it is impossible to obtain uniform cooling over the entire strand width.
A further accommodation of the cooling, especially for metallurgical reasons, can be realized by changing the pressure in the infeed lines or conduits leading to the nozzles. Such pressure changes bring about changes in the angle of the spray pattern or spray jet, resulting in undesired changes in the size of the impinged surface at the strand and thus the cooling action.